Will Boulder’s Water Supply Stand Up to Climate Change? (Part 1)

Boulder Creek. Photo: Chris Carruth

The 2002 drought in Boulder, Colorado, sparked concerns about how a warming climate could impact the city’s water supply. After assessing historical streamflow records, tree ring data, and climate change models, a team of researchers concluded that even under future climate change scenarios water shortages don’t appear imminent. However, with runoff from the Rocky Mountains predicted to increase during spring and decrease during summer, over the coming decades, climate change could create complications for water rights holders in future.

In this three-part series, Caitlyn Kennedy, science writer for NOAA’s Climate Program Office, explains how climate change is expected to impact Boulder’s water supply.

(This story originally ran at NOAA’s ClimateWatch, June 3, 2011).

Nestled in the foothills of the Rocky Mountains is the city of Boulder, Colorado, where Carol Ellinghouse has worked for over two decades as the city’s water resources coordinator. On countless walks along wandering mountain trails that overlook the scenic valley, her eyes have skimmed the pockets of green that mark the locations of the city’s many parks and open spaces, and in the distance to the east, fields of corn, alfalfa and pasture grass.

Located in the semi-arid western United States, Boulder Valley wouldn’t look so green naturally.

“We get an average of about 15 inches of rain and snow in the city every year,” Ellinghouse explains. “For lawn grass to grow, you probably need somewhere around 30 to 40 inches.”

Fortunately, it snows much higher up in the mountains west of Boulder. All of the farmers, residents, and wild creatures living in the Boulder Valley and downstream in the South Platte River basin compete to use the meltwater that trickles down from the high country in the spring and summer.

Boulder is located at the base of the foothills of the Rocky Mountains at an elevation of 5,430 feet (1,655 m). On average from year to year, the city’s annual water supply comes from Barker Reservoir on Middle Boulder Creek, Silver Lake/Lakewood Watershed on North Boulder Creek, and the Boulder Reservoir. (Map by Hunter Allen, using elevation data from USGS, GIS layers for Boulder County, and Landsat imagery from March 2009.)

Over the past century, Boulder has built a system of reservoirs and pipelines that capture runoff from melting mountain snowpack, and every year Ellinghouse and her co-workers ensure that a continuous flow of water reaches the city’s treatment plants.

But after an exceptional drought in 2002, Boulder’s leaders became concerned that its water system would become insufficient in the future. Climate change, they worried, could change the rules of the game.

Members of the community began peppering Ellinghouse with questions. How often are droughts as severe as the 2002 event likely to occur? Will the snowpack decline? Are water shortages going to become more common? Is Boulder’s water system resilient enough to handle a hotter and potentially drier climate?

Ellinghouse knew that some of the information she needed to answer these questions was already available, starting with local climate and stream flow observations that extend back about 100 years. She also knew that she could locate scientists who could extend the local water history far back in time by analyzing growth rings in centuries-old trees.

But getting some insight into what the future holds for Boulder would be a lot more challenging. Global climate models project how different levels of carbon dioxide and other greenhouse gases in the atmosphere are likely to affect global or regional climate patterns, but not local ones.

To evaluate the water system, Ellinghouse needed to answer very specific questions: how will climate change impact the way Boulder residents use water in their homes and backyards? How will it affect the fish swimming in Boulder Creek? And what about local firefighters who need thousands of gallons of water to battle dangerous, fast-moving wildfires?

What she needed was help building a bridge between the large-scale projections of climate models and the detailed history of local climate provided by both modern streamflow and weather records and tree-ring measurements.

Boulder Droughts: Not if, but When

“Any issue involving water is a hot topic when you’re living in a semi-arid climate,” Ellinghouse says with a slight chuckle. “There’s simply more demand for water than there is water available.”

Water availability in any year depends on how much snowfall the Rocky Mountains receive that winter. West of Boulder, snow accumulates to an average depth of 50 inches each winter. This snowpack usually contains between 9-30 inches of water, depending on how many snowstorms pass through in a year.

The streamflow in Boulder Creek—the valley’s primary water source—peaks in late spring or summer, as mountain snowpack melts. Flows decline dramatically by late summer and remain low into winter. As a result, Ellinghouse and her team are very dependent on the city’s reservoirs to store water and even everything out.

Many factors dictate how much water Boulder can use directly from the stream or store in its reservoirs every year—legal water rights, natural swings in streamflows and precipitation, irrigation demand, and population growth. Occasional water shortages are inevitable.

“It’s not a matter of if, it’s a matter of when,” Ellinghouse says. “Droughts are something we plan for, and they’re something we expect to deal with occasionally. It’s just a part of the natural cycle in Colorado.”

To protect the city’s water supply from crippling droughts, Ellinghouse first looks at extremes—exceptionally wet or dry years—in the observed streamflow record, and then she adds a margin of safety to the amount of water carried over in city reservoirs from wet years for dry year use.

The observed streamflow record goes back about 100 years. In a climate as variable as Colorado’s, even a 100-year record does not give her great insight into the kinds of droughts that may have occurred in the more distant past.

The tree rings of the Douglas-fir tree are very sensitive to changes in moisture conditions during the tree’s lifetime, making it ideal for reconstructing climate records. A dry year produces a narrow growth ring, and a wet year produces a wider one. (Photo credit: H.D. Grissino-Mayer, University of Tennessee.)

For a longer history of Colorado’s wet and dry extremes, Ellinghouse relies on the expertise of scientists who reconstruct past climates, or “paleoclimates,” using tree growth rings. Tree growth rings reveal swings in moisture; a dry year leads to a narrow growth ring, and a wet year leads to a wide growth ring.

Scientists can use the trees’ measurable “moisture signals” to reconstruct the history of how much water flowed through streams, rivers, and creeks at different times in the past. Because streamflow is directly influenced by precipitation and temperature, variability in streamflow reflects climate variability.

In 2000, Ellinghouse hired Lee Rozaklis and his team of water resource consultants at AMEC, an environmental consulting firm (formerly known as Hydrosphere Resources Consultants, Inc.) to collaborate with the City of Boulder on a study that would examine the city’s drought vulnerability to a repeat of the past 300 years of climate variability. They also worked with Connie Woodhouse at the University of Arizona, who used tree ring data to reconstruct Boulder Creek stream flows back to 1703. (A few years later, this record was extended to cover 1566-2003.)

Variability in streamflow reconstructed from tree-ring data for Boulder Creek near Orodell, CO, shows the 2002 drought was comparable to the lowest flow in the 437-year record. The reconstructed series represents the statistically “most-likely” annual flow for each year. The 2002 drought was one of the few years in the record that the volume of streamflow dipped below 30,000 acre-feet per year. (Data provided by Connie Woodhouse, University of Arizona.)

The timing of the study was quite fortunate. AMEC helped the City of Boulder recognize and respond to the 2002 drought—the most extreme drought in Boulder’s 100-year stream gage record. According to the paleoclimate record, the 2002 drought produced the lowest stream flow seen since the mid 1800s.

In a sense, the analysis meant that Boulder had already been thrown one of Mother Nature’s worst curveballs and had handled it without significant water shortages. Usually, during a drought of this severity, the city would have to severely restrict outside water use; landscaping would be expected to suffer. Instead, voluntary conservation allowed the city to sustain nearly 60 percent of normal outdoor watering needs, with only minor losses to landscaping.

Although this was certainly good news, Ellinghouse knew she also had to look ahead. What might the 2002 drought look like in the future, magnified by a climate that was warmer and potentially drier overall?

Check in tomorrow for part 2.





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